1. Field of the Invention
This invention relates to wavefront phase imaging methods to include interferometric phase imaging. More specifically, the invention is a method that collects multiple wavefront phase images with known sub-pixel shifts being implemented between each image, and combines these images into a single image of the measured wavefront.
2. Description of the Related Art
It is well known that all optical systems and their components produce optical wavefronts. Examples of components include reflecting surfaces, refracting surfaces, transmitting surfaces, diffracting surfaces, light emitting surfaces, etc. Examples of systems using these components include telescopes, camera and lithography lenses, lasers, etc. The measurement of optical wavefronts is extremely useful and provides valuable information about the physical properties or performance of individual optical components or the assembled system. The measurement of optical wavefronts is well known and includes the following techniques/systems:
interferometry,
phase-measuring interferometers or interference microscopes employing any number of different methods such as phase-stepping, phase-shifting, instantaneous, carrier fringe, etc.,
use of other wavefront sensors employing any number of different methods such as phase-diversity, phase-retrieval, lens-let arrays, screen tests, Ronchi tests, etc.
All of these wavefront measurement methods provide data over a limited spatial frequency band.
Generating high-resolution images of an optical system's wavefront phase is useful for a variety of applications. For example, technical specifications for high-precision optical components include distinct spatial frequency bands over the full aperture of an optical component. Interferometers are used to perform measurements of an optical component in order to provide information on these specifications. The spatial frequency of the surface features that an interferometer can resolve is defined by the Nyquist frequency of the imaging systems or 1/(2*pixel size). That is, an interferometer cannot resolve spatial frequencies that are greater than this limit. Thus and as is well known in the art, an interferometer's optical resolution is limited by the pixel size and spacing of the interferometer's “change coupled device” (CCD) detector/imager. For example, if an optical component has a full aperture of 100 millimeters, an interferometer having a 1024×1024 CCD detector (i.e., a square array of 1024 pixels by 1024 pixels) can resolve features that are 0.2 millimeters or larger. However if the same CCD detector is used to generate an interferogram of an optical component having a full aperture of 4 meters, this same interferometer cannot resolve any features that are smaller than 8 millimeters. Since the performance of a 4 meter high-precision optical component can easily be compromised by a feature that is less than 8 millimeters, it is necessary to make higher resolution interferograms. Note that the same discussion also applies to smaller optical components. For example, the Nyquist limit affects the ability of interference microscopes to measure high-spatial frequency features such as surface roughness, physical phenomena, natural structure, engineered micro or nano-scale structure, etc.
Current approaches to solving the Nyquist problem include the use of higher resolution CCD detectors and sub-aperture imaging where the CCD detector takes multiple sub-aperture sized images of an optical component and then “stitches” the sub-aperture images together. However, higher resolution CCD detectors greatly increase the cost of interferometric measurement systems. The sub-aperture imaging approach requires fairly large movements of the optical component that must be precise in order to enable the interferometer to “stitch” the multiple images together accurately. The motion and time between sub-aperture measurements can cause measurement “noise” that decreases the accuracy of the measurement data. Further, stitching requires the overlap of data sets. To minimize the propagation of low spatial frequency noise in the stitching result, typical processing techniques use a data overlap of 50% that, in turn, drives the number of measurements needed to reconstruct the wavefront phase.